1. Field of the Invention
The invention relates to an image forming apparatus of an electrophotographic system such as a copying machine, laser beam printer, or the like.
2. Related Background Art
As an image forming apparatus, FIG. 8 shows a schematic construction of an example of a multicolor image forming apparatus for forming a color image by using an intermediate transfer member. The multicolor image forming apparatus is a copying machine or a laser beam printer using an electrophotographic process. The construction and the operation of the image forming apparatus will now be simply explained hereinbelow.
As shown in FIG. 8, an electrophotographic photosensitive member (photosensitive drum) 1 of a rotary drum type serving as an image bearing member is arranged in a main body of the image forming apparatus. The photosensitive drum 1 is rotated at a predetermined peripheral speed (processing speed) in a direction shown by an arrow R1. In the rotating step, the surface of the photosensitive drum 1 is charged to a predetermined electric potential of a predetermined polarity by a charging unit 2 such as a corona discharging unit or the like. The charged surface of the drum 1 is subjected to an image exposure L by an exposing device 3 (an image formation exposure optical system based on a color separation of a color original image, a scanning exposure optical system by a laser scanner to generate a laser beam modulated in correspondence to a time sequential electric digital image signal of image information). Thus, an electrostatic latent image corresponding to a color separation image of a first color, for example, a magenta component image of a target color image is formed.
Subsequently, the latent image is developed by, for example, a first developing unit (magenta developing unit) 4a among four developing units 4 using magenta toner (colored charging particles of magenta) and is visualized as a magenta toner image. The magenta toner image formed on the photosensitive drum 1 is transferred onto the surface of an intermediate transfer drum 50 serving as an intermediate transfer member.
The intermediate transfer drum 50 is constructed by forming an elastic layer 52 of a middle resistance onto a conductive base layer 51 and by further forming a mold releasing layer onto the elastic layer 52. As an intermediate transfer member, a belt-shaped drum can be used in addition to a drum shape like an intermediate transfer drum 50.
The intermediate transfer drum 50 comes into contact with the photosensitive drum 1 and is rotated in a direction shown by an arrow R2 at the same speed as that of the photosensitive drum 1. At the time of the transfer, a primary transfer bias of a polarity (plus) opposite to a toner charging polarity (minus in this example) of the toner image on the photosensitive drum 1 is applied to the base layer 51 by a primary transfer bias power source 61. The magenta toner image of the first color formed on the photosensitive drum is transferred onto the surface of the intermediate transfer drum 50 by applying the transfer bias (primary transfer).
After completion of the transfer of the toner image, the surface of the photosensitive drum 1 is cleaned by a cleaning device 14, thereby removing the transfer residual toner remaining on the surface.
Similarly, the charging is performed to the photosensitive drum 1, the image exposure L corresponding to a color component image of the second color, for example, cyan component image, and the development of an electrostatic latent image by a second developing unit 4b (cyan developing unit) are performed, thereby obtaining a cyan toner image of the second color. The obtained cyan toner image is transferred onto the magenta toner image formed on the surface of the intermediate transfer drum 50. The surface of the photosensitive drum 1 to which the cyan toner image was transferred is cleaned by the cleaning device 14.
In a manner similar to the above, with respect to the third and fourth colors, for example, yellow and black, the image exposure L to the photosensitive drum 1, the development of electrostatic latent images by a third developing unit 4c (yellow developing unit) and a fourth developing unit 4d (black developing unit), and an overlap transfer of obtained yellow toner image and black toner image onto the surface of the intermediate transfer drum 50 are performed.
By sequentially executing the forming and transferring processes of the toner images of four colors as mentioned above, a color toner image (having a mirror image relation with an original color image) in which the toner images of four colors of magenta, cyan, yellow, and black were overlapped is formed as a synthetic color image corresponding to a target color image onto the surface of the intermediate transfer drum 50.
One transfer material (paper) P is taken out from a sheet feeding cassette 9 and is conveyed. The transfer material P is fed to a transfer unit formed by a transfer charging unit (corona charging unit) 7 and the intermediate transfer drum 50 at a predetermined timing through a registration roller pair 11 and a transfer guide 12.
By applying a transfer bias of a plus polarity opposite to the charging polarity of the toner to the transfer charging unit 7 by a secondary transfer bias power source 71, the toner images of four colors on the intermediate transfer drum 50 are transferred onto the surface of the transfer material P in a lump while the transfer material P passes through a transfer portion (secondary transfer).
The transfer material P to which the toner images of four colors were transferred is led to a fixing unit 15 from the intermediate transfer drum 50 via a conveying guide 13. In the fixing unit 15, the toner images of four colors are heated and pressed and subjected to a fixing process by a fixing roller 16 heated to a predetermined temperature and a fixing roller 17 which comes into pressure contact with the fixing roller 16. A final full color image is formed and, after that, the transfer paper is discharged to the outside of the image forming apparatus.
After completion of the transfer of the toner images, the transfer residual toner remaining on the surface of the intermediate transfer drum 50 is cleaned and removed by a cleaning device 8. The cleaning device 8 is arranged so as to freely come into contact with and removed from the intermediate transfer drum 50. When the transfer of the toner images of the intermediate transfer drum 50 is finished, the cleaning device is removed from a normal separate state to an operative state where the transfer paper is in contact with the surface of the intermediate transfer drum 50.
To keep a density of the color image on the transfer material P at a desired predetermined density, a density sensor is arranged near the photosensitive drum 1. For example, when a power source of the image forming apparatus main body is turned on or when a predetermined number of images are formed or the like, a plurality of patch patterns of a predetermined size and each color are formed as toner images onto the photosensitive drum 1 by changing an image forming condition such as a developing bias. A density of the patch of each color is detected by the density sensor, thereby obtaining the image forming condition to realize the optimum density from a change in density of the patch. A density control to change to such a condition is performed.
Although the patch patterns are dither patterns in many cases, the other various patterns are also used. There is also a method whereby a density sensor is arranged near the intermediate transfer drum 50 and the density of the patch transferred onto the intermediate transfer drum is read, thereby performing the density control.
However, when the above density control is performed, there is a case where the following problems occur.
When a multicolor image in which image portions of different colors are arranged is formed after completion of the density control, there is a case where a phenomenon (white gap) such that although no gap exists in adjacent boundary portions of image portions which are neighboring on image data, it seems as if a white line existed there occurs.
For example, FIG. 9 shows a state of a case where an image portion Qm of magenta and an image portion Qc of cyan are arranged in parallel and formed. According to the ideal image data, the image portions Qm and Qc should be neighboring without any gap as shown in FIG. 9A. However, a gap q due to the white gap is formed at a boundary between them as shown in FIG. 9B.
This phenomenon also occurs in a portion where image portions of different colors and arbitrary shapes are neighboring without being limited to the case where the rectangular image portions of different colors are adjacent as mentioned above. In a natural image, graph chart, or the like, when a white line due to the white gap appears, an image quality remarkably deteriorates.
As another problem, a fogged image on background or the like occurs because characteristics of developers of developing units, particularly, color developing units (developing units 4a, 4b, and 4c of magenta, cyan, and yellow) are changed in association with the image formation. When the fogged image on background is generated, the image is formed as if the whole image became dirty and the image quality is also remarkably deteriorated.
The above various phenomena are caused due to a potential contrast (contrast potential, back contrast potential) between a latent image potential (dark portion potential, light portion potential) on the surface of the photosensitive drum where the image formation is performed and a developing potential, an aging change in an environment of the developers, an aging change due to the use for a long period of time, and further, a developing system. Generally, the white gap occurs when the back contrast potential is large and the fogged image on background occurs when the back contrast potential is small.